base: fix accessibility of enum in bit_array.h

avoids warning by clang:

error: 'BITS_PER_WORD' is a private member of 'Genode::Bit_array_base'

Issue #3022

repos/base/include/util/bit_array.h

@@ -35,13 +35,15 @@ class Genode::Bit_array_base
 		class Invalid_clear        : public Exception {};
 		class Invalid_set          : public Exception {};
 
-	private:
+	protected:
 
 		enum {
 			BITS_PER_BYTE = 8UL,
 			BITS_PER_WORD = sizeof(addr_t) * BITS_PER_BYTE
 		};
 
+	private:
+
 		unsigned _bit_cnt;
 		unsigned _word_cnt;
 		addr_t  *_words;